In this program, we will use the strstr() function. This function is used to find the first occurrence of a specified substring within the string and return the pointer to the calling function. It returns NULL if the substring is not found within the string.

Program:

The source code to demonstrate the strstr() function is given below. The given program is compiled and executed using GCC compile on UBUNTU 18.04 OS successfully.

// C program to demonstrate the strstr() function

#include <stdio.h>
#include <string.h>

int main()
{
    char str[32] = "India is great country";
    char* ptr;

    ptr = strstr(str, "great");
    if (ptr != NULL)
        printf("String is: '%s'\n", ptr);
    else
        printf("Word 'great' is not found\n");

    ptr = strstr(str, "power");
    if (ptr != NULL)
        printf("String is: %s\n", ptr);
    else
        printf("Word 'power' is not found\n");

    return 0;
}

Output:

String is: 'great country'
Word 'power' is not found

Explanation:

In the main() function, we created a string str initialized with "India is great country". Then we used the strstr() function to find the specified substring "reat" within the string. The strstr() function is used to find the first occurrence of a specified substring within the string and returns the pointer to the calling function. It returns NULL if the substring is not found within the string.